1. Field of the Invention
The invention generally relates to a phase-locked loop apparatus (PLL apparatus) and a tuning voltage providing circuit thereof, and more particularly, to an apparatus able to accelerate phase-locking operations of a PLL apparatus by using a tuning voltage providing circuit.
2. Description of Related Art
In order to provide a periodic signal with a stable phase, the PLL apparatus is the most often employed and the most reliable one. The PLL apparatus can effectively tune the phase of the output signal produced by itself according to the phase of an input signal, so that the PLL apparatus can serve as a frequency multiplier or a spread spectrum circuit able to dynamically and finely tune the frequency of the output signal thereof.
In a conventional PLL apparatus, affected by a capacitor with large capacitance in a loop filter of the apparatus, the phase-locking operation often costs a long time to be finished. In order to solve the above-mentioned problem, a scheme has been proposed, in which a control voltage of a voltage-controlled oscillator (VCO) in the PLL apparatus is preset in advance to a level closer to a stable level at which the PLL apparatus finishes the phase-locking operation. However, different circuit processes, different environment temperatures and different power voltage ranges in applications would affect the stable level of the control voltage. Thus, it is difficult to appropriately set the initial level of the control voltage.
In addition, another conventional approach is also used, by employing a charge pump that generates a larger charging current to perform a charging operation before the PLL apparatus finishes a frequency/phase-locking operation, and decreases the charging current after the PLL apparatus finishes the frequency/phase-locking operation. However, this approach would bring out many new problems such as an increased circuit area of the charge pump for providing a larger charging current, and difficulty in obtaining precise time points for switching the charging current.